Final answer:
When light from a star has a longer wavelength than expected, it typically indicates that the star is moving away from us, a conclusion based on the Doppler effect and known as redshift.
Step-by-step explanation:
If light from a star is observed to have a longer wavelength than expected, the most likely explanation is that the star is moving away from the observer. This phenomenon is a result of the Doppler effect, where the wavelength of light or other waves elongates when the source of the waves is receding from the observer. When applied to astrophysical observations, this shift towards longer wavelengths is known as redshifting. In the case of a star or galaxy moving away at high speeds, spectral lines characteristic of elements such as hydrogen will appear at longer wavelengths than those measured in a laboratory. Thus, when we see the light from a star redshifted, it is evidence of its recession from us.